spi: restore rx/tx_buf in case of unset CONFIG_HAS_DMA

The case where spi_master sets the flags SPI_MASTER_MUST_RX/TX while
CONFIG_HAS_DMA is unset (which is unlikley) together with a driver
that reuses spi_messages with rx/tx_buff set to NULL, can result in:
* data disclosure over the SPI (for tx_buf == NULL)
* memory corruption (for rx_buf == NULL)

This happenes when dummy_rx/dummy_tx are changing address due to krealloc
or free and an allocation of the memory by a different part of the kernel.

Signed-off-by: Martin Sperl <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi.c

···
 	return 0;
 }
 
-static int spi_unmap_msg(struct spi_master *master, struct spi_message *msg)
+static int __spi_unmap_msg(struct spi_master *master, struct spi_message *msg)
 {
 	struct spi_transfer *xfer;
 	struct device *tx_dev, *rx_dev;
···
 	rx_dev = master->dma_rx->device->dev;
 
 	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
-		/*
-		 * Restore the original value of tx_buf or rx_buf if they are
-		 * NULL.
-		 */
-		if (xfer->tx_buf == master->dummy_tx)
-			xfer->tx_buf = NULL;
-		if (xfer->rx_buf == master->dummy_rx)
-			xfer->rx_buf = NULL;
-
 		if (!master->can_dma(master, msg->spi, xfer))
 			continue;
 
···
 	return 0;
 }
 
-static inline int spi_unmap_msg(struct spi_master *master,
-				struct spi_message *msg)
+static inline int __spi_unmap_msg(struct spi_master *master,
+				  struct spi_message *msg)
 {
 	return 0;
 }
 #endif /* !CONFIG_HAS_DMA */
+
+static inline int spi_unmap_msg(struct spi_master *master,
+				struct spi_message *msg)
+{
+	struct spi_transfer *xfer;
+
+	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
+		/*
+		 * Restore the original value of tx_buf or rx_buf if they are
+		 * NULL.
+		 */
+		if (xfer->tx_buf == master->dummy_tx)
+			xfer->tx_buf = NULL;
+		if (xfer->rx_buf == master->dummy_rx)
+			xfer->rx_buf = NULL;
+	}
+
+	return __spi_unmap_msg(master, msg);
+}
 
 static int spi_map_msg(struct spi_master *master, struct spi_message *msg)
 {